Nonequilibrium Molecular Dynamics Simulations of n-Butane and Isobutane Viscosity

Abstract

Nonequilibrium molecular dynamics simulations of Lennard-Jones site-site models representing n-butane and isobutane were performed over much of the density range for which experimental viscosity data are available. Simulated viscosities extrapolated to zero shear agreed very well with experimental data over the entire density range. The shear perturbs the equilibrium structure of the fluid and produces shear birefringence or molecular alignment. The relative ability of the molecules to orient in the shear field accounts for their relative shear-thinning rheology. Saturation of the shear birefringence produces a change in the observed rheology at higher shear rates. The effect of shear on thermodynamic properties is also investigated.

Key Words:

• Molecular dynamics
• n-butane
• isobutane
• viscosity
• nonequilibrium molecular dynamics
• structured molecules
• site-site model